It has been known in the past that acrylic block copolymers or acrylic block copolymer compositions are used for pressure-sensitive adhesives. For example, a composition comprising a specific block copolymer having a structure represented by the general formula: -[a1]-[b]-[a2] in the polymer main chain and a diblock copolymer containing at least one (meth)acrylic acid alkyl ester polymer block is excellent not only in various pressure-sensitive adhesion properties such as holding power at high temperatures but also in heat resistance, weathering resistance and hot-melt coating properties and is preferable as a pressure-sensitive adhesive (particularly, hot-melt type pressure-sensitive adhesive) for obtaining pressure-sensitive adhesive sheets and pressure-sensitive adhesive tapes (see patent literature 1).
It is disclosed that a diblock copolymer, which is represented by the general formula: X-Y (in this formula, X is a polymer block mainly constituted of a methacrylic acid alkyl ester unit having an alkyl group of 1 to 4 carbon atoms or an alkyl group having a cyclic structure, and Y is a polymer block mainly constituted of an acrylic acid alkyl ester unit having an alkyl group of 1 to 20 carbon atoms and/or a methacrylic acid alkyl ester unit having an alkyl group of 5 to 20 carbon atoms) and in which the weight-average molecular weight (Mw) of the polymer block X is 1,000 to 8,000 and the ratio of the mass of the polymer block X to the mass of the polymer block Y, that is, X/Y mass ratio, is in the range of 1/99 to 10/90, is useful for obtaining an acrylic pressure-sensitive adhesive that is excellent not only in heat resistance, weathering resistance and various pressure-sensitive adhesion properties but also in hot-melt coating properties (see patent literature 2).
It is disclosed that by blending a block copolymer which has in a molecule at least an acrylic triblock structure wherein one or two acrylic acid ester-based polymer blocks and two or one (meth)acrylic acid ester-based polymer block having a different structure are bonded and which has a weight-average molecular weight (Mw) of not less than 120,000 and a molecular weight distribution of less than 1.5, with a diblock copolymer in which one acrylic acid ester-based polymer block and one (meth)acrylic acid ester-based polymer block having a different structure are bonded and which has a molecular weight distribution of less than 1.5, in a specific proportion, an acrylic block copolymer composition excellent in heat resistance, weathering resistance, bleed resistance, hot-melt properties, melt coating properties, and various pressure-sensitive adhesion properties, such as adhesive strength, cohesive force and tack, is obtained. It is described that various pressure-sensitive adhesion properties are further improved by setting the content of the (meth)acrylic acid ester-based polymer block in the block copolymer constituting the acrylic block copolymer composition to 5 to 22% by mass or by setting the weight-average molecular weight (Mw) of the block copolymer to not less than 150,000 (see patent literature 3).
However, it has not been specifically studied to apply the pressure-sensitive adhesive compositions disclosed in the above patent literatures 1 to 3 to pressure-sensitive adhesion type optical films. With regard to properties of solution type pressure-sensitive adhesives containing acrylic triblock copolymer, anything has not been mentioned at all.
On the other hand, pressure-sensitive adhesives for optical films are used for applying optical films such as polarizing plates and retardation films to liquid crystal panels or applying plural optical films to one another to laminate them or applying protective films to optical films.
In particular, the pressure-sensitive adhesives for optical films are required to have excellent transparency in addition to excellent performance balance as pressure-sensitive adhesives, and besides, they are required to be improved in reworkability, heat resistance, durability, etc.
Specifically, the pressure-sensitive adhesives for optical films are required to have not only transparency but also excellent optical isotropy (no birefringence) in order that the functions of optical films should not be impaired.
In the application of an optical film, an optical film or a protective film once applied is sometimes peeled off and applied again if wrinkles, bubbles, inclusion of foreign matters, deviation of application position, or the like occurs, or after peeling of the optical film, an expensive liquid crystal panel is sometimes recovered and recycled. From such viewpoints, the pressure-sensitive adhesives for optical films are required to have initial reworkability such that there is no adhesive transfer in the process for producing an image display device such as a display, peeling can be carried out with proper peel strength, and re-application is possible.
Further, the pressure-sensitive adhesives for optical films are required to have high durability such that even if the pressure-sensitive adhesive is heated or exposed to moist heat after application of an optical film and before actual use of the optical film, the pressure-sensitive adhesive does not undergo foaming or does not cause peeling of the film from an adherend.
A member comprising an optical film, a base and a pressure-sensitive adhesive layer containing an acrylic block copolymer is known (see patent literature 4). In this literature, however, initial reworkability necessary for pressure-sensitive adhesives for optical films has not been studied, though an effect on inhibiting ply separation and foaming in the pressure-sensitive adhesive layer has been studied in the case where the base is a gas releasing base such as polycarbonate or poly(methylmethacrylate).
Under such circumstances as above, the present inventors have developed a non-chemical crosslinking type pressure-sensitive adhesive for optical films containing a specific acrylic triblock copolymer as a main component, and previously applied for a patent (see patent literature 5). This non-chemical crosslinking type pressure-sensitive adhesive for optical films is free from a problem of variability in pressure-sensitive adhesion performance due to uneven crosslinking, exhibits good cohesive force, is excellent in initial reworkability, pressure-sensitive adhesion properties, heat resistance, durability, etc. and is useful as a pressure-sensitive adhesive for usual optical films. However, even this non-crosslinking type pressure-sensitive adhesive does not have sufficient initial reworkability and durability in some cases.